Long-term biomechanical stability of self-expanding stents using the crossover technique for vertebral artery ostium stenosis: A tortuosity index analysis

Abstract

Background:

Vertebral artery ostium (VAO) stenosis treatment with balloon-expandable stents is challenging due to precise positioning difficulties and high rates of restenosis and fracture. Self-expanding stents (SES) using the crossover technique from VAO to proximal subclavian artery may offer better accessibility, though long-term outcomes remain unclear.

Methods:

Ten patients with symptomatic VAO stenosis underwent SES placement using the crossover technique, extending from V1 segment to proximal subclavian artery. Tortuosity index was measured from digital subtraction angiography frontal projections using automated analysis with manual correction at pre-procedure, post-procedure, and >3-year follow-up. Clinical and angiographic outcomes were evaluated.

Results:

Technical success was 100%. Mean tortuosity index improved from 1.15 ± 0.09 to 1.03 ± 0.03 post-procedure (P < 0.01) and remained stable at 1.02 ± 0.02 at mean 4.1-year follow-up. No restenosis occurred. One fracture (10%) occurred at an intentionally balloon-dilated site for future radial access; no unintended mechanical failures occurred (0/9).

Conclusion:

SES crossover technique demonstrates excellent long-term structural integrity with stable vessel geometry and zero unintended fractures over >3 years in VAO treatment.

INTRODUCTION

Posterior circulation stroke accounts for 20–25% of ischemic strokes, with up to one-third attributable to severe vertebral artery ostium (VAO) stenosis.[10] Despite optimal medical therapy, symptomatic VAO stenosis carries a 30% stroke risk at 5 years, warranting endovascular intervention.[5] Historically, balloon-expandable stents (BES) borrowed from coronary and renal applications dominated VAO treatment due to high radial strength and precise deployment. However, this precision is often challenged by subclavian artery angulation and tortuosity. Moreover, restenosis rates remain problematic, with chronic stent fracture reported in 6.5–15% of cases.[12,13]

Self-expanding stents (SESs) offer theoretical advantages through superelasticity, shape memory, and superior flexibility. Previous reports of SES use in VAO showed inconsistent results. Open-cell type stents present positioning challenges at the subclavian artery junction,[1] while closed-cell types demonstrate stent shortening and delayed malposition issues.[6] Li et al. reported favorable outcomes with SES deployment from V1 to proximal subclavian artery without fractures.[7] However, quantitative assessment of long-term vessel geometry stability has not been reported.

This study examines long-term structural integrity of SES using this crossover strategy in symptomatic VAO stenosis, with particular focus on device durability and vessel geometry stability over >3 years follow-up.

MATERIALS AND METHODS

Patient selection

Between 2010 and 2019, 82 patients underwent VAO stenting at our institution. All were symptomatic with vertebrobasilar stroke, transient ischemic attack (TIA), or syncope with contralateral vertebral artery (VA) compromise. Twelve patients received crossover technique, with 10 completing >3-year angiographic follow-up. Written informed consent was obtained from all patients, and the study was approved by the hospital ethics committee. Dual antiplatelet therapy was initiated at least five days before the procedure.

Procedure

Procedures utilized femoral access with proximal protection (8-French balloon guide catheter) or distal filter protection. SES was deployed from V1 segment to proximal subclavian artery. Stent selection was based on intravascular ultrasound (IVUS) findings: Tapered Protégé (Medtronic Minneapolis, MN, USA) was preferred, with 3–4 mm balloon pre-dilation. For cases without diameter mismatch between VA and subclavian artery, non-tapered stents (Protégé or Precise, Cordis, Miami Lakes, FL, USA) were selected. Post-dilation was performed with balloon size below the medial diameter of VAO on IVUS. Dual antiplatelet therapy was maintained for 6 months, followed by single antiplatelet therapy indefinitely. Digital subtraction angiography (DSA) evaluation was performed after >3 years.

Tortuosity index measurement

Degree of stenosis and Tortuosity Index was calculated using automated measurement with manual correction (Image J, Fuji Film, Tokyo, Japan). Degree of stenosis was calculated by most stenotic lesion diameter/distal normal V1 segment of the VA diameter. Tortuosity index was calculated as follows. Pre-procedure measurements assessed the future stent site centerline; post-procedure measurements assessed the stent centerline. The index equals centerline length divided by straight-line distance between endpoints [Figure 1]. A tortuosity index of 1.0 indicates a perfectly straight path, whereas greater degrees of curvature are associated with progressively higher index values.

Figure 1:

Tortuosity index measurement and changes. (a) Pre-procedure angiography showing vertebral artery origin stenosis. The white line indicates centerline measurement of the future stent placement site. Tortuosity Index (TI) equals centerline length divided by straight-line distance between endpoints; TI was 1.24. (b) Post-procedure angiography. TI was measured using the stent centerline divided by straight-line distance between endpoints; TI was 1.06. (c) Six-year follow-up maintaining stable configuration; TI was 1.03.

Statistical analysis

Continuous variables are presented as mean ± standard deviation. Paired t-test compared pre- and post-procedure values. P < 0.05 was considered significant.

RESULTS

Overall cohort outcomes

Among 82 VAO lesions, 70 received focal VAO stenting (52 renal artery BES, 13 coronary BES, and 5 precise SES). Follow-up imaging (angiography or computed tomography angiography) at ≥6 months in 46 cases revealed 3/46 (6.5%) stent fractures and 3/46 (6.5%) restenosis, all asymptomatic.

Among 12 crossover SES cases, 10 underwent DSA evaluation >3 years post-procedure and were analyzed in detail.

Baseline characteristics

Ten patients (8 males, mean age 71.0 ± 5.5 years) with symptomatic VAO stenosis (mean 78.0 ± 13.5%) were included. Risk factors included hypertension (60%), diabetes (30%), and hyperlipidemia (50%). Mean follow-up was 4.1 ± 1.0 years [Table 1].

Table 1:

Patient demographics, procedural details, and long-term outcomes.

Procedural outcomes

Proximal protection with 8-French balloon guide catheter was used in nine cases, distal filter protection in one case. Technical success was 100%. Stents used were tapered Protégé (n = 6) and non-tapered Protégé or Precise (n = 4). Mean post-dilation diameter was 4.9 mm. No periprocedural complications occurred. Residual stenosis was <20% in all cases.

In one patient with multiple coronary stenoses, after crossover stenting from V1 to proximal subclavian artery, intentional balloon dilation (4.0 mm) was performed through stent struts from proximal to distal subclavian artery to facilitate future transradial coronary access. No immediate fracture was observed [Figure 2].

Figure 2:

Intentional fracture case. (a) Pre-procedure angiography showing vertebral artery origin stenosis; Tortuosity Index was 1.19. (b) Fluoroscopy during post-stent balloon dilation with 5.5 mm balloon after self-expanding stent placement. (c) Post-procedure angiography showing good reconstruction; TI was 1.02. (d) Intentional balloon dilation (4.0 mm) through stent struts from proximal VAO to distal subclavian artery to facilitate future transradial coronary access; no immediate fracture observed. (e) Three-year follow-up angiography showing localized fracture at the intentionally dilated segment without clinical sequelae (arrow indicates fracture site).

Tortuosity index evolution

Tortuosity index significantly improved from 1.15 ± 0.09 pre-procedure to 1.03 ± 0.03 post-procedure (P < 0.01), remaining stable at 1.02 ± 0.02 at follow-up (P = NS vs. post-procedure). The mean change from post-procedure to follow-up was 0.7% (absolute value), indicating exceptional geometric stability [Figure 3].

Figure 3:

Changes in tortuosity index over time.

Clinical and angiographic outcomes

No vertebrobasilar strokes, TIAs, or vascular deaths occurred. Restenosis rate was 0%. One stent fracture (10%) occurred at the intentionally balloon-dilated site [Figure 2], remaining asymptomatic. Importantly, no unintended mechanical failures occurred (0/9 patients).

DISCUSSION

Biomechanical superiority of the crossover approach

Our findings demonstrate exceptional long-term durability of SES using the crossover technique, with zero unintended fractures over >3 years. This contrasts sharply with BES studies reporting 10–15% fracture rates in VAO[12] and validates the biomechanical principles underlying our approach.

The VAO represents a unique biomechanical challenge, experiencing both cervical motion-induced flexion/torsion and upper extremity motion-induced axial strain. Studies demonstrate vertebral artery strain reaches 12–18% during neck rotation, approaching the 16–20% elongation threshold for arterial rupture.[3]

Stents placed at VAO experience forces through two mechanisms: (1) pulsatile and shear stress: the ostium is a bifurcation point exposed to significant elastic forces and pulsations from both subclavian and vertebral arteries.[7] (2) Torsional and bending stress: head and neck movements cause continuous flexion, extension, and torsion of implanted devices, leading to mechanical fatigue.[8] Traditional BES placement creates a “hinge point” at stent edges where the rigid device meets dynamic vessel, concentrating stress and precipitating both mechanical failure (fracture) and biological failure (restenosis through chronic irritation).

The superior durability of SES is attributed to nitinol’s superelasticity, allowing natural conformity to tortuous vessel anatomy and demonstrating “crush recoverability” when subjected to external forces, enabling continued function without permanent deformation. Precise and Protégé stents feature laser-cut open-cell design with fewer inter-strut connections, providing high flexibility and excellent conformability to tortuous vessels. Stent fractures occur most frequently at connector curves where stress concentration occurs. Placing BES in the highly mobile VAO creates stress concentration points at hinge points, potentially leading to fracture. In contrast, the SES crossover technique distributes these hinge points, reducing fracture risk.[8] In addition, the tapered design of Protégé prevents localized stress concentration from diameter mismatch between small-caliber vertebral artery and large-caliber subclavian artery.[7]

A significant anatomical consideration in VAO treatment is the inherent vessel diameter discrepancy between the VA (typically 3–5 mm) and subclavian artery (7–10 mm). Tapered stent design provides optimal radial force distribution by delivering adequate support at the larger subclavian segment while avoiding excessive compression at the vertebral segment. Two tapered Protégé stent configurations are available: 7 mm distal/10 mm proximal and 6 mm distal/8 mm proximal. Our protocol requires IVUS measurement of the subclavian artery landing zone, with stent diameter selected to exceed the measured vessel diameter, ensuring adequate wall apposition while minimizing excessive radial force. This approach reduces chronic outward radial pressure compared to non-tapered stents, which necessitate uniform oversizing throughout their entire length and potentially cause excessive compression forces at the smaller VA segment.

Comparative biomechanics

Our outcomes align with SES performance in other mobile, non-compressed arteries. Iliac arteries show 3.6–5% fracture rates with no impact on patency.[4] Carotid arteries demonstrate 3.4% fracture rates, typically asymptomatic due to collateral circulation.[2] In contrast, femoropopliteal arteries experience 33–65% fracture rates with direct correlation to restenosis, reflecting extreme mechanical stress from compression and angulation.[9] VAO treated with appropriate technique behaves like carotid rather than femoropopliteal arteries – mobile but not compressed, with adequate collateral circulation mitigating minor mechanical failures.

Tortuosity index stability

The remarkable stability of tortuosity index (0.7% change over 3 years) provides quantitative evidence of achieved biomechanical equilibrium. This stability indicates: (1) absence of chronic straightening forces, (2) no progressive stent deformation from cyclic loading, and (3) minimal vessel wall irritation (correlating with 0% restenosis). This geometric stability represents true integration between device and vessel, the hallmark of optimal biomechanical design.

Study limitations

This single-center retrospective study includes only ten patients without randomized controls. Larger prospective trials comparing crossover technique to standard BES placement are warranted. Tortuosity index measurements were performed only on frontal DSA projections without 3D data analysis. In addition, respiratory variation effects on vessel deviation were not assessed.[11]

CONCLUSION

SES placement using the crossover technique demonstrates exceptional long-term structural integrity for symptomatic VAO stenosis, with zero unintended failures and stable vessel geometry over >3 years. This approach represents a paradigm shift from pursuing anatomical precision to achieving biomechanical harmony, addressing the unique challenges of this dynamic vascular territory.

Ethical approval:

The research/study approved by the Institutional Review Board at Fukuoka Wajiro Hospital, number No. 00233, dated April 03, 2025.

Declaration of patient consent:

The authors certify that they have obtained all appropriate patient consent.

Financial support and sponsorship:

Nil.

Conflicts of interest:

There are no conflicts of interest.

Use of artificial intelligence (AI)-assisted technology for manuscript preparation:

The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.

Disclaimer

The views and opinions expressed in this article are those of the authors and do not necessarily reflect the official policy or position of the Journal or its management. The information contained in this article should not be considered to be medical advice; patients should consult their own physicians for advice as to their specific medical needs.